1. Field of the Invention
This invention generally relates to all-reflective optical systems for broadband wafer inspection. Certain embodiments relate to a system that includes an optical subsystem of which all light-directing optical components are reflective optical components except for one or more refractive optical components that are located in substantially collimated space such that the refractive optical component(s) do not introduce aberrations to the light. The reflective optical components are located in non-collimated space and substantially collimated space.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
One obvious way to improve the detection of relatively small defects is to increase the resolution of an optical inspection system. One way to increase the resolution of an optical inspection system is to decrease the wavelength at which the system can operate. As the wavelength of inspection systems decrease below the visible waveband, relatively good resolution can be easily achieved for relatively small wavebands. For example, optical components that have relatively low aberrations across small wavebands are relatively easy to design and fabricate.
However, it is also desirable for inspection systems to be able to operate over a relatively broad waveband such that defect detection can be performed across a broad range of wavelengths or a number of smaller bandwidths selectable from a larger bandwidth for a given circumstance. Having the capability to generate inspection data across a broad waveband has obvious benefits such as increasing the flexibility of the inspection system for performing inspection of a wide variety of wafers that have substantially different characteristics. In addition, inspection data generated across a broad waveband may provide significantly more information about a wafer than inspection data generated at only a single wavelength or across a narrow waveband. Furthermore, thin-film interference between transparent layers on a semiconductor wafer can create measurement noise because of variations in the thickness of the layers. If a broadband light source is used to illuminate the wafer, the interference between the different wavelengths tends to balance out thereby minimizing the interference noise effect. Thus, having a broadband illumination source is desirable to minimize noise thereby allowing smaller defects to be seen.
Designing a broadband inspection system that can operate at wavelengths below the visible waveband is not a trivial matter. However, some optical inspection systems have been designed for broadband wafer inspection below the visible waveband using only refractive lenses, refractive lenses formed of a combination of both fused silica and calcium fluoride (CaF2) elements, or catadioptric lens assemblies that include a mixture of both fused silica and CaF2 elements. Fused silica in combination with a material such as CaF2 or a mirror/lens arrangement that includes elements formed of such a combination of materials are used in inspection system lenses to overcome the dispersion of light by fused silica thereby making an inspection system having a significant bandwidth possible.
The material combinations described above, however, typically cannot be used for a broadband optical system designed to operate at wavelengths below 230 nm. For example, the transmission of fused silica drops dramatically below about 185 nm due to absorption of the light by the oxygen (O2) molecules in the fused silica. Therefore, the transmission properties of fused silica prohibit any broadband solution below that wavelength as there are not two dissimilar refractive materials available for use below that wavelength. Even above that wavelength, the dispersion of fused silica becomes relatively large at wavelengths approaching the absorption edge (See FIG. 2). Therefore, the achievable waveband below about 230 nm is rather narrow using a lens that includes fused silica elements. For example, typically a waveband of only 10 nm to 20 nm in the wavelength range around 200 nm is achievable using inspection systems that include fused silica elements in combination with CaF2 in a catadioptric configuration. When the dispersion of fused silica flattens out at longer wavelengths, then a longer waveband is feasible.
Examples of lenses that include fused silica elements and are configured for microscopic inspection across a waveband from 230 nm to 370 nm are illustrated in U.S. Pat. No. 5,999,310 to Shafer et al., which is incorporated by reference as if fully set forth herein. The lenses described in this patent provide significant wafer inspection capability as well as advantages such as the ability to use different types of illumination and minimization of aberrations such as coma, astigmatism, primary color aberrations, and residual lateral color aberrations. Therefore, although significant utility in wafer inspection applications has been found for these lenses, due to the transmission properties of fused silica described above, these lenses are not suited for broadband wafer inspection below wavelengths of 200 nm or even 230 nm. These limitations are not specific to the lenses described in this patent but will be true for any inspection system that includes at least one lens element that is formed of fused silica.
Accordingly, it would be advantageous to develop a wafer inspection system that provides wafer inspection capability across a broad waveband that includes wavelengths below and above 200 nm.